This invention relates to an output sealing structure of a magnetron, and more particularly to an improved joint between the magnetron and a waveguide which forms a portion of a microwave oven, for example, or a support of the magnetron.
Such a magnetron is used in a microwave oven, for example, as diagrammatically shown in FIG. 1. As shown, the microwave oven 11 comprises a cooking chamber 14 for accommodating a foodstuff 13 to be cooked and a door 12. An opening 15 is formed through the upper wall of the cooking chamber 14 for admitting the microwave from the output 21 of a magnetron 20 through a waveguide 16, thereby cooking the foodstuff by dielectric heating.
In such an oven, it is necessary to transmit all microwaves radiated from the output 21 through the waveguide 16, but if the seal at the joint between the output 21 and the waveguide 16 were not perfect a portion of the microwave would leak to the outside thus decreasing the efficiency, injuring the user and causing spark across the gap at the joint.
To overcome these difficulties, the seal at the output 21 of the magnetron has been constructed as shown in FIG. 2 which shows a magnetron 20 of the outside magnet type. The magnetron 20 illustrated comprises upper and lower rectangular yokes 25 and 26, cylindrical permanent magnets 27 and 28 between opposite ends of the yokes, an anode cylinder 30 and sector shaped ducts 31 and 32 (the latter is not shown in FIG. 2) projecting in the radial direction from the anode cylinder 30. At the center of the yoke 25 is formed an opening 33 through which the output portion 21 of the magnetron comprising an antenna 34 and a ceramic insulator 35 extends to the outside. Similar opening (not shown) is also provided at the center of the yoke 26 for passing a cathode structure including heater terminals, cathode stem, etc.
As shown, yoke 25 is connected directly to a flange 42 and waveguide 16 by means of bolts 40 and 41. The flange 42 is used as a support for the magnetron and is provided with a circular opening 44 at its center having an edge 45 bent toward the yoke 25. The waveguide 16 is secured to the flange 42 to surround the opening 44. The pole pieces 45' project at the base of the output portion 21 and an annular metal gasket 26 surrounds the pole pieces. In this example, pole pieces 45' and gasket 46 are contained in the opening 33 of yoke 25. The inturned edge 45 and the gasket 46 have substantially the same diameter so that the inturned edge 45 is caused to urge against the gasket when the yoke 25 is connected to the flange 42 and the waveguide 16 by bolts 40 and 41 whereby the pole pieces 45', gasket 46, flange 42 and waveguide 16 are electrically interconnected and the gaps between these elements are sealed to prevent leakage of the microwave.
With such sealing structure, however, due to oxidation of the surface of the gasket 46, or decrease in the resiliency thereof, the electrical and mechanical contacts between the inturned edge 45 and the gasket 46 become imperfect thus causing increase in the contact resistance or leakage of the microwave or electric spark.
Furthermore, the centering of the waveguide 16 and the magnetron can be performed only by suitable selection of relative position of openings 47 and 48 (not shown) and the bolts 40 and 41. As a consequence, it is necessary to make the diameter of the openings 47 and 48 to be minimum so as to minimize the clearance between the inner surfaces thereof and the bolts 40 and 41. Further, the positions of the bolts should be accurately determined thus requiring careful machining and assembling.